LTC6104I View Datasheet(PDF) - Linear Technology
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Description
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LTC6104I Datasheet PDF : 16 Pages
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LTC6104
APPLICATIONS INFORMATION
Selection of External Current Sense Resistor
The external sense resistor, RSENSE, has a significant effect
on the function of a current sensing system and must be
chosen with care.
First, the power dissipation in the resistor should be
considered. The system load current will cause both heat
and voltage loss in RSENSE. As a result, the sense resis-
tor should be as small as possible while still providing
the input dynamic range required by the measurement.
Note that input dynamic range is the difference between
the maximum input signal and the minimum accurately
reproduced signal, and is limited primarily by input DC
offset of the internal amplifier of the LTC6104. In addition,
RSENSE must be small enough that VSENSE does not exceed
the maximum input voltage specified by the LTC6104, even
under peak load conditions.
As an example, an application may require that the
maximum sense voltage be ±100mV. If this application
is expected to draw ±2A at peak load, RSENSE should be
no more than 50mΩ.
RSENSE
=
VSENSE
IPEAK
=
100mV
2A
=
50mΩ
Once the maximum RSENSE value is determined, the mini-
mum sense resistor value will be set by the resolution or
dynamic range required. The minimum signal that can be
accurately represented by this sense amp is limited by the
input offset. As an example, the LTC6104 has a typical
input offset of ±85µV. If the minimum current is ±20mA, a
sense resistor of 4.25mΩ will set VSENSE to ±85µV. This is
the same value as the input offset. A larger sense resistor
will reduce the error due to offset by increasing the sense
voltage for a given load current.
Choosing a 50mΩ RSENSE will maximize the dynamic range
and provide a system that has ±100mV across the sense
resistor at peak load (±2A), while input offset causes an
error equivalent to only ±1.7mA of load current. Peak dis-
sipation in the sense resistor is 200mW in this example.
If instead a 5mΩ sense resistor is employed, then the ef-
fective current error is ±17mA, while the peak sense voltage
is reduced to ±10mV at ±2A, dissipating only 20mW.
The low offset and corresponding large dynamic range of
the LTC6104 make it more flexible than other solutions
in this respect. The ±85µV typical offset gives 60dB of
dynamic range for a sense voltage that is limited to ±85mV
max, and over 75dB of dynamic range if the rated input
maximum of ±500mV is allowed.
Sense Resistor Connection
Kelvin connection of the –INA/–INB and +INA/+INB in-
puts to the sense resistor should be used in all but the
lowest power applications. Solder connections and PC
board interconnections that carry high current can cause
significant error in measurement due to their relatively
large resistances. One 10mm × 10mm square trace of
one-ounce copper is approximately 0.5mΩ. A 1mV error
can be caused by as little as 2A flowing through this small
interconnect. This will cause a 1% error in a 100mV signal.
A 10A load current in the same interconnect will cause
a 5% error for the same 100mV signal. By isolating the
sense traces from the high current paths, this error can
be reduced by orders of magnitude. A sense resistor with
integrated Kelvin sense terminals will give the best results.
Figure 2 illustrates the recommended method.
ILOAD
– VSENSE +
TO
CHARGER/LOAD
RSENSE
+
RIN
RIN
8
+INA
A
VS
7
6
–INA –INB
5
+INB
–+
B
VS
CURRENT
OUT
LTC6104
MIRROR
V–
1
4
+
VOUT
–
ROUT
+– VREF
6104 F02
Figure 2. Kelvin Input Connections Preserve Accuracy
Despite Large Load Currents
6104f
8
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